The present invention relates to a method for measuring the concentration of impurities in nitrogen, hydrogen and oxygen by means of ion mobility spectrometry.
Nitrogen, hydrogen and oxygen are some of the gases used as reaction media, or as the actual reagents, in the integrated circuits industry. It is known that in the production of these devices the purity of the reagents is of utmost importance; as a matter of fact, contaminants possibly present in the reagents or in the reaction environment can be incorporated into the solid state devices, thus altering the electrical properties thereof and giving rise to production wastes. The specifications on the purity of the gases employed in production can vary from one manufacturer to another and depend on the particular process in which the gas is employed. Generally, a gas is considered to be acceptable for production when its impurities content is not higher than 10 ppb (parts per billion); preferably, the content in impurities is lower than 1 ppb. As a result, it is important to be able to measure extremely low concentrations of impurities in gases in a precise and reproducible way.
A technique which can be used for this purpose is ion mobility spectrometry, known in the field with the abbreviation IMS; the same abbreviation is also used for the instrument with which the technique is carried out, in this case indicating “Ion Mobility Spectrometer.” The interest for this technique derives from its very high sensitivity, associated with the limited size and cost of the instrument. By operating under appropriate conditions it is possible to sense species in the gas or vapor phase in a gaseous medium in quantities on the order of picograms (pg, 10−12 grams), or in concentrations on the order of parts per trillion (ppt, equivalent to one molecule of analyzed substance for every 10−12 gas molecules of the sample). IMS instruments and methods of analysis in which they are employed are described, for example, in U.S. Pat. Nos. 5,457,316 and 5,955,886 in the name of the U.S. company PCP Inc, and in U.S. Pat. No. 6,229,143, in the name of the Applicant.
An IMS instrument is essentially formed of a reaction zone, a separation zone and a collector of charged particles.
In the reaction zone takes place the ionization of the sample comprising the gases or vapors to be analyzed in a carrier gas, commonly by means of beta-radiation emitted by 63Ni. The ionization takes place mainly on the carrier gas with the formation of the so-called “reagent ions,” whose charge is then distributed onto the species present depending on their electron or proton affinities or their ionization potentials.
The reaction zone is divided from the separation zone by a grid which, when kept at a suitable potential, prevents the ions produced in the reaction zone from entering into the separation zone. The moment when the grid potential is annulled, thus allowing the ions to enter into the separation zone, is the “time zero” of the analysis.
The separation zone comprises a series of electrodes which create an electric field such that the ions are carried from the reaction zone toward a collector. In this zone, which is kept at atmospheric pressure, a gas flow having opposite direction with respect to that of the ions' movement is present. Commonly, the counterflow gas, defined in the field as “drift gas,” is an extremely pure gas corresponding to the gas whose content of impurities is to be determined. As an example, in an IMS analysis for determining the content of impurities in nitrogen, the drift gas is normally pure nitrogen. The velocity of motion of the ions depends on the electric field and on the cross-section of the same ions in the gaseous medium, so that different ions take different times for crossing the separation zone and for reaching the particle collector. The time passed from the “time zero” to the time of arrival on the particle collector is called “time of flight.” The collector is connected to a signal processing system, which transforms the current values sensed as a function of time into a final graph in which peaks corresponding to the various ions as a function of the “time of flight” are shown. From the determination of this time and knowing the test conditions, it is possible to determine the presence of the substances which are the object of the analysis. From the peak areas with suitable computation algorithms, it is also possible to calculate the concentrations of the corresponding species.
In spite of its conceptual simplicity, the application of the technique involves some difficulties in the interpretation of the analysis results. This is due firstly to the fact that the net charge distribution among the various species present is the result of equilibria which depend on various factors, with the result that the peaks corresponding to one impurity can be modified in intensity, or even disappear, depending on the presence of other impurities. The book “Ion Mobility Spectrometry” by G. A. Eiceman and Z. Karpas, published in 1994 by CRC Press, can be referred to for an illustration of the (rather complex) charge transfer principles which are the base of the technique. Further, keeping constant the chemical composition of the gas, the results depend on the analysis parameters, such as the electric field applied in the separation zone, the flow rate of the gas which has to be analyzed, and the flow rate of the drift gas.
As a consequence of these phenomena, the shape of the graph resulting from an IMS analysis is strongly dependent on the analysis conditions. The computation algorithms used for interpreting the analysis results are based on the deconvolution of the complete graph and on the relative measure of the areas of all the peaks present. The best results are obtained when each ionic species present gives rise to a separate peak in the graph. The analysis is still possible, although with greater difficulties, when the time of flight of a limited number of different species are similar, giving rise to a few peaks derived from the superimposition of singular peaks; in these cases it is necessary to resort to hypotheses about how the peak area is to be shared among the different species, with the risk however of introducing errors in the analysis. Finally, the IMS analysis (also the qualitative one) is impossible when large superimpositions between peaks corresponding to different species occur.
Because of the complexity of the phenomena in play, there is no standard method for applying the IMS technique, and each analysis has to be studied separately in order to define the conditions which allow for a good separation of all the peaks corresponding to the different species which can be present in the gas under analysis.